merge_trees(): add ancestor label parameter for diff3-style output

Commands using the merge_trees() machinery will present conflict hunks
in output something like what ‘diff3 -m’ produces if the
merge.conflictstyle configuration option is set to diff3.  The output
lacks the name of the merge base on the ||||||| line of the output,
and tools can misparse the conflict hunks without it.  Add a new
o->ancestor parameter to merge_trees() for use as a label for the
ancestor in conflict hunks.

If o->ancestor is NULL, the output format is as before.  All callers
pass NULL for now.

If o->ancestor is non-NULL and both branches renamed the base file
to the same name, that name is included in the conflict hunk labels.
Even if o->ancestor is NULL I think this would be a good change, but
this patch only does it in the non-NULL case to ensure the output
format does not change where it might matter.

Requested-by: Stefan Monnier <monnier@iro.umontreal.ca>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

merge-recursive.h

@@ -4,6 +4,7 @@
 #include "string-list.h"
 
 struct merge_options {
+	const char *ancestor;
 	const char *branch1;
 	const char *branch2;
 	enum {